7.11 JS Strict Mode

The "use strict" Directive

• The "use strict" directive was new in ECMAScript version 5.
• It is not a statement, but a literal expression, ignored by earlier versions of JavaScript.
• The purpose of "use strict" is to indicate that the code should be executed in "strict mode".
• With strict mode, you can not, for example, use undeclared variables.
• All modern browsers support "use strict" except Internet Explorer 9 and lower:

Directive
"use strict"	13.0	10.0	4.0	6.0	12.1

• The numbers in the table specify the first browser version that fully supports the directive.
• You can use strict mode in all your programs. It helps you to write cleaner code, like preventing you from using undeclared variables.
• "use strict" is just a string, so IE 9 will not throw an error even if it does not understand it.

The "use strict"; Syntax

• The syntax, for declaring strict mode, was designed to be compatible with older versions of JavaScript.
• Compiling a numeric literal (4 + 5;) or a string literal ("John Doe";) in a JavaScript program has no side effects. It simply compiles to a non existing variable and dies.
• So "use strict"; only matters to new compilers that "understand" the meaning of it.

Why Strict Mode?

• Strict mode makes it easier to write "secure" JavaScript.
• Strict mode changes previously accepted "bad syntax" into real errors.
• As an example, in normal JavaScript, mistyping a variable name creates a new global variable. In strict mode, this will throw an error, making it impossible to accidentally create a global variable.
• In normal JavaScript, a developer will not receive any error feedback assigning values to non-writable properties.
• In strict mode, any assignment to a non-writable property, a getter-only property, a non-existing property, a non-existing variable, or a non-existing object, will throw an error.

Not Allowed in Strict Mode

Using a variable, without declaring it, is not allowed:

"use strict";
x = 3.14;                // This will cause an error

Try it Yourself

• Objects are variables too.
• Using an object, without declaring it, is not allowed:

"use strict";
x = {p1:10, p2:20};      // This will cause an error

Try it yourself

Deleting a variable (or object) is not allowed.

"use strict";
var x = 3.14;
delete x;                // This will cause an error

Try it yourself

Deleting a function is not allowed.

"use strict";
function x(p1, p2) {};
delete x;                // This will cause an error 

Try it yourself

Duplicating a parameter name is not allowed:

"use strict";
function x(p1, p1) {};   // This will cause an error

Try it yourself

Octal numeric literals are not allowed:

"use strict";
var x = 010;             // This will cause an error

Try it yourself

Octal escape characters are not allowed:

"use strict";
var x = "\010";            // This will cause an error

Try it yourself

Writing to a read-only property is not allowed:

"use strict";
var obj = {};
Object.defineProperty(obj, "x", {value:0, writable:false});

obj.x = 3.14;            // This will cause an error

Try it yourself

Writing to a get-only property is not allowed:

"use strict";
var obj = {get x() {return 0} };

obj.x = 3.14;            // This will cause an error

Try it yourself

Deleting an undeletable property is not allowed:

"use strict";
delete Object.prototype; // This will cause an error

Try it yourself

The word eval cannot be used as a variable:

"use strict";
var eval = 3.14;         // This will cause an error

Try it yourself

The word arguments cannot be used as a variable:

"use strict";
var arguments = 3.14;    // This will cause an error

Try it yourself

The with statement is not allowed:

"use strict";
with (Math){x = cos(2)}; // This will cause an error

Try it yourself

For security reasons, eval() is not allowed to create variables in the scope from which it was called:

"use strict";
eval ("var x = 2");
alert (x);             // This will cause an error

Try it yourself

• The this keyword in functions behaves differently in strict mode.
• The this keyword refers to the object that called the function.
• If the object is not specified, functions in strict mode will return undefined and functions in normal mode will return the global object (window):

"use strict";
function myFunction() {
  alert(this); // will alert "undefined"
}
myFunction();

Try it yourself

Future Proof!

• Keywords reserved for future JavaScript versions can NOT be used as variable names in strict mode.
• These are:
  • implements
  • interface
  • let
  • package
  • private
  • protected
  • public
  • static
  • yield

"use strict";
var public = 1500;      // This will cause an error

Try it yourself

• Watch Out!
• The "use strict" directive is only recognized at the beginning of a script or a function.